from my immediate subject. (Dr. Pavy's "Treatise on Food" has an introductory chapter on "The Dynamic Relations of Food," in which this subject is clearly treated in sufficient detail for popular reading.)
It is quite the fashion now to rely upon these later experiments; but, for my own part, I am by no means satisfied with them—and for this reason, that the perspiration from the skin and the vapor from the lungs were not examined. It is just these which are greatly increased by exercise, and their quantity is very large, especially those from the skin, which are threefold, viz., the insensible perspiration, which is transpired by the skin as invisible vapor; the sweat, which is liquid; and the solid particles of exuded cuticle.
Lavoisier and Seguin long ago made very laborious experiments upon themselves in order to determine the amount of the insensible perspiration. Seguin inclosed himself in a bag of glazed taffeta, which was tied over him with no other opening than a hole corresponding to his mouth; the edges of this hole were glued to his lips with a mixture of turpentine and pitch. He carefully weighed himself and the bag before and after his inclosure therein. His own loss of weight being partly from the lungs and partly from the skin, the amount gained by the bag represented the quantity of the latter; the difference between this and the loss of his own weight gave the amount exhaled from the lungs.
He thus found that the largest quantity of insensible exhalation from the lungs and skin together amounted to three and a half ounces per hour, or five and a quarter pounds per day. The smallest quantity was one pound fourteen ounces, and the mean was three pounds eleven ounces. Three fourths of this was cutaneous.
These figures only show the quantity of insensible perspiration during repose. Valentin found that his hourly loss by cutaneous exhalation while sitting amounted to 32·8 grammes, or rather less than one and a quarter ounce. On taking exercise, with an empty stomach, in the sun, the hourly loss increased to 89·3 grammes, or nearly three times as much. After a meal followed by violent exercise, with the temperature of the air at 72° Fahr., it amounted to 132·7 grammes, or nearly four and a half times as much as during repose. A robust man, taking violent exercise in hot weather, may give off as much as five pounds in an hour.
The third excretion from the skin, the epithelial or superficial scales of the epidermis, is small in weight, but it is solid, and of similar composition to gelatine. It should be understood that this increases largely with exercise. The practice of sponging and "rubbing down" of athletes removes the excess; but I am not aware of any attempt that has been made to determine the quantity thus removed.
Does the skin excrete nitrogenous matter that may be, like urea, a product of the degradation or destruction of muscular tissue?
